1. Field of the Invention
The present invention relates to electroluminescent devices, and more specifically, to electroluminescent devices with enhanced lifetime.
2. Description of the Related Art
Conventionally, an electroluminescent (EL) light source comprises two congruently disposed electrodes, one of them being transparent within the optical spectral region, with dielectric and electroluminescent layers located in-between. Applying alternating voltage with corresponding frequency and amplitude, for example, within the ranges of 102 Hz–104 Hz, 30 V–300 V, to the electrodes, the electroluminescent layer starts emitting light throughout the transparent electrode. In the course of operation, degradation processes occur in the light-emitting layer, which lead to a decrease in the brightness of the EL light source and a simultaneous growth of its impedance. Sometimes this process is called aging of the EL light source. The lifetime of the EL light source is defined as the time of continuous luminescence with the luminescence brightness decreasing down to half of the initial brightness, known as the half-decay time. The higher the frequency and the amplitude of the input voltage, the higher is the initial brightness of the EL light source. However, in this case its lifetime is shorter than in the case of applying input voltage with a lower frequency and amplitude.
Standard EL light source drivers are known and in use, which are described, for instance, in the U.S. patents listed in Table 1:
TABLE 1U.S. Pat. No.Issue DatePatenteeClass/Subclass3,749,977Jul. 31, 1973Sliker315/2834,254,362Mar. 3, 1981Tulliners315/2194,611,150Sep. 9, 1986Ball et al.315/3074,633,141Dec. 30, 1986Weber315/307with each of U.S. Pat. Nos. 3,749,977; 4,254,362; 4,611,150; and 4,633,141 being incorporated herein by reference.
The devices disclosed in U.S. Pat. Nos. 3,749,977; 4,254,362; 4,611,150; and 4,633,141 are DC-to-AC inverters for converting DC voltage into AC voltage with a specified frequency and amplitude, and with an output signal shape close to a sinusoidal shape. The transformer of the DC-to-AC inverter determines the output voltage and also plays a role in determining the frequency. An EL light source together with a DC-to-AC inverter constitute a compensating integrated circuit. The capacitance of the EL light source coupled with the inductance of the secondary coil of the transformer constitute a tunable LC circuit. The DC-to-AC inverter functions as a resonating oscillator compensating for the decreasing capacitance of an aging EL light source. As the EL light source ages, the DC-to-AC inverter compensates for the decrease in the output brightness by increasing its output voltage and frequency. The use of DC-to-AC inverters described above increases the lifetime of EL light sources by about 20%–30% in comparison with the lifetime of the EL light source fed with an input AC voltage with frequency and amplitude unchanging in time.
A much more efficient method of compensating for the processes of EL light source aging was proposed in U.S. Pat. No. 5,440,208, issued Aug. 8, 1995 to Uskali et al., in class/subclass 315/169; with U.S. Pat. No. 5,440,208 being incorporated herein by reference.
According to U.S. Pat. No. 5,440,208, a driver is disclosed which involves a feedback circuit comprising a voltage drop divider using a resistor connected in series with the EL light source. The voltage drop on this resistor is changed in the process of the EL light source aging. The voltage from this divider is fed to one of the inputs of a pulse generator, so that as the EL light source ages, pulse amplitude increases. A serious drawback of the proposed solution is that it involves a high initial dissipated power, and besides, the shape of the generated signal is far from being sinusoidal, which intensifies the degradation processes in the EL light source.
Several types of drivers are known with a high-frequency sinusoidal signal fed in packets. As a rule, such drivers are used to feed multi-segment EL light sources, as described in the U.S. patents listed in Table 2:
TABLE 2U.S. Pat. No.Issue DatePatenteeClass/Subclass4,238,793Dec. 9, 1980Hochstrate345/1024,253,097Feb. 24, 1981Hochstrate345/1024,449,075May 15, 1984D'Onofrio et al.315/196with each of U.S. Pat. Nos. 4,238,793; 4,253,097; and 4,449,075 being incorporated herein by reference.
FIG. 1 shows an electrical schematic diagram of one of several types of available DC-to-AC inverters 10 for EL light sources in the prior art. If the selection of resistors 12, 14 and transistor 26 is correct, the inverter 10 generates a quasi-sinusoidal signal. The parameters of a resonant circuit 19, formed by a secondary inductance 22 of the transformer 23 and capacitance of the EL light source 32, determine the frequency of the output signal of DC-to-AC inverter 10. The turn ratio of the transformer 23 and the current generated in an auxiliary coil 28 wound on the core 24 of the transformer 23 determine the amplitude of the signal generated by the DC-to-AC inverter 10. In the process of the EL light source operation, as a result of complicated physical processes of degradation or aging, its luminescence brightness decreases with a simultaneous increase in the impedance of the EL light source, mainly due to the capacitance decrease. In this situation, the parameters of the oscillatory resonant circuit 19 and, to a lesser extent, the current generated in the coil 28, are changed. Due to these changes, the frequency and amplitude of the quasi-sinusoidal signal at the output of the DC-to-AC inverter 10 increase. This results in the increase of luminescence brightness, which partly compensates for the brightness decrease caused by the light source aging. Respectively, the lifetime of the EL light source increases in comparison with the lifetime of a similar EL light source fed by AC voltage with constant values of the sinusoidal signal frequency and amplitude. FIG. 9 shows a plot of the dependence of the luminescence brightness of the EL light source on light emitting time for various types of drivers. Curve 78 relates to the supply of AC voltage with parameters that remain unchanged in time, whereas curve 80, which relates to the supply of AC voltage using the DC-to-AC inverter 10 as shown by the schematic diagram, is presented in FIG. 1. Here the initial values of the EL light source brightness are the same in both cases. It is evident that the EL light source lifetime in curve 80 is generally about 30% longer than the lifetime represented by curve 78.